RF signals used for digital data transmission standards (for example, signals having a frequency in the range from 3 kHz to 300 GHz) are often characterized by high peak power-to-average power ratios. An amplifier for such signals should be capable of amplifying the peak amplitude in the same conditions as the average amplitude. However, in practice, the amplifier most of the time processes signals close to the average amplitude.
If the amplifier comprises a single output stage operating, to have a good linearity, in class A or AB, the quiescent current of the amplifier is proportional to the current that it should provide for peak amplitudes. When the amplifier processes signals of lower amplitude, that is, most of the time, the quiescent current of the output stage, and thus its power consumption, are unnecessarily high, which results in a poor general efficiency.
To improve the efficiency, it has already been provided to form an amplifier comprising a main output stage configured to operate in class A or AB, having its quiescent current adjusted to linearly amplify signals of medium power, and an auxiliary output stage configured to operate in class B or C, that is, to only amplify the portion of the input signal which exceeds a given power threshold. The auxiliary stage is set to start amplifying when the main stage starts saturating. As long as the turn-on threshold of the auxiliary stage has not been reached, this stage consumes no current. The sum of the outputs of the two amplification stages then corresponds to the signal that the main stage would provide if it did not saturate.
FIG. 1 is a simplified diagram illustrating this type of operation in further detail. More particularly, FIG. 1 illustrates the variation of power Pout of the output signal according to power Pin of the input signal. When power Pin is lower than a threshold T, main amplification stage A1 operates in a linear area of a transfer curve H(A1) (in full line in the drawing). When input power Pin exceeds threshold T, the main amplification stage starts saturating, as shown by the flattening of curve H(A1), and auxiliary amplification stage A2 starts operating according to a transfer curve H(A2) (in full line in the drawing). The main stage then generates a clipped output signal, and the auxiliary stage only amplifies the portion of the input signal which exceeds threshold T. General output power Pout is formed by the sum of the outputs of the two amplification stages. Its variation is shown by a transfer curve H(A1+A2) (in dotted lines). The auxiliary output stage also ends up saturating, which is shown as a flattening of curves H(A2) and H(A1+A2). Thus, the gain compression, and then the saturation, of main amplification stage A1, of class A or AB, are compensated by the gain expansion of auxiliary amplifier A2, of class B or C, to obtain, by combination, a substantially linear response across an extended power range.
An example of an amplifier with two output stages operating in different input signal amplitude ranges is particularly described in article “Transformer-Based Uneven Doherty Power Amplifier in 90 nm CMOS for WLAN Applications” (Solid-State Circuits, IEEE, vol. 47, no 7, pp. 1659-1671, July 2012, incorporated by reference).
More generally, many amplifiers of this type, generally called Doherty amplifiers, have already been provided.
A disadvantage of existing architectures of multi-range amplifiers is that they generally comprise power division circuits at the input of the assembly of output stages and/or power combiner circuits at the output of the assembly of output stages. Such circuits have the disadvantage of being relatively bulky, and of generating power losses at least partially compensating the efficiency gain provided by the association of a plurality of output stages having different operating ranges.
In practice, in existing architectures, the number of output stages with different operating ranges capable of being associated in a same amplifier is limited to two, particularly due to the losses and to the bulk associated with the power division and/or combiner circuits. The general efficiency of the amplifier thus remains relatively poor.